If you are sea kayaking or sailing, always wear a buoyancy aid and ensure it is fastened properly. Especially with children, who can slip out of them easily.
If you are hiking, let someone know where you are going and when you plan to be back.
Since the end of the wars in 1995, there are still thousands of landmines in Croatia. Not in tourist areas, thankfully, and areas which are suspected to have mines are marked with mine warning signs. So, talk to a local if you are going off the beaten path
Watch out for Bura wind warning signs, mostly in the Velebit area and in the winter months.
The coasts are very rocky, so rubber shoes are a must, and also carry antiseptic spray and plasters to the beach, as cuts do happen.